World leaders have vowed to boost intelligence-sharing, cut off terrorist funding and strengthen border security in Europe as they sought to show resolve and unity following the Paris terror attacks.

German Chancellor Angela Merkel said: "We agreed that the challenge can't just be tackled with military means, but only a multitude of measures."

The leaders of the Group of 20 rich and developing nations were wrapping up their two-day summit in Turkey against the backdrop of heavy French bombardment of Islamic State's stronghold in Syria.

The US is to expand its intelligence sharing with the French and help them identify targets, according to American officials.

Numerous meetings about next steps in Syria and the Islamic State campaign were being held on the sidelines of the summit in the Turkish seaside resort of Antalya.

US president Barack Obama met with European leaders from France, Britain, Germany and Italy. French president Francois Hollande skipped the summit to stay home and deal with the aftermath of the attacks, but Foreign Minister Laurent Fabius planned to attend the meeting with the US president.

Russian president Vladimir Putin, a key player in ending the conflict in Syria that created a vacuum for IS, met separately with Italian prime minister Matteo Renzi and Prime Minister David Cameron.

Mr Putin and Mr Cameron both emphasised the need for joint action against terrorism in the wake of the attacks.

"The latest tragic events in Paris show that we have to unite our efforts in fighting this evil, something we should have done a long time ago," Mr Putin said.

He launched an air campaign in Syria around six weeks ago with IS the top declared target.

The US and its allies, however, have accused Moscow of focusing on other rebel groups in a bid to shore up Syria's leader Bashar Assad, whom the West sees as the main cause of the Syrian conflict and the chief obstacle to peace.

Ahead of the G-20, foreign ministers met in Vienna to discuss a new diplomatic plan to end the Syrian war. The plan appears to be based largely on a Russian proposal that envisions negotiations between Mr Assad's government and opposition groups starting by January 1.

Still, sharp differences over Assad's future and disagreements about what militant groups in Syria should be considered terrorists have dampened hopes for a breakthrough.

The Vienna talks were high on the agenda when Mr Obama and Mr Putin met for about 35 minutes on Monday on the sidelines of the G-20.

Mr Obama appeared to take a softer tone with Mr Putin in the talks, noting "the importance of Russia's military efforts" aimed at IS, according to a White House readout of the meeting.

His spokesman Dmitry Peskov said it would be "absolutely unrealistic" to expect a brief Obama-Putin meeting to mark a turning point in bilateral ties.

"The bilateral relations are as they are. There are differences, but there is understanding that there is no alternative to bilateral dialogue to discuss the existing problems," Mr Peskov told reporters.

Mr Obama also met with Saudi Arabia's King Salman on Sunday, another key player in the effort to end the Syrian war.

The Saudis have pushed strongly for the ouster of Mr Assad and have funded his foes. Mr Putin is also set to have talks with the Saudi king, who is scheduled to visit Moscow soon.

Amid the diplomatic wrangling, the shock over the Paris attacks that killed at least 129 people raised the demand for quick action.

The attacks, along with earlier bombings in Lebanon and Turkey and the downing of a Russian airliner in Egypt, indicated that IS has grown bold enough to strike a variety of targets far away from its base in Syria and Iraq.

In addition to the violence and instability afflicting much of the world, this year's G-20 agenda also included efforts to hasten global economic growth, with a particular focus on addressing the effects of China's economic slowdown.

In a draft of the final G-20 communique obtained by The Associated Press, leaders renewed their goal to grow their collective GDP by another 2% by 2018.